Microwave band-pass filter having constant bandwidth as filter is tuned

ABSTRACT

In a filter comprising a plurality of cavities coupled to each other by means of metal probes, the cavities have respective tuning rods which extend therein and whose cross-sectional area is not constant throughout the whole of their length, so that the filter bandwidth is made fairly constant within a large range of tuning frequencies.

United States Patent Moreau 1 July 25, 1972 MICROWAVE BAND-PASS FILTER 1 r nc s Cited HAVING CONSTANT BANDWIDTH AS UNITED STATES PATENTS FILTER IS TUNED 3,353,122 11/1967 Manoochehri ..333/73 W [72] Inventor: Jean-Noel Moreau, Paris, France 3,428,918 2/1969 Matthei 1 h k.'. [73] Assignee: Thomson-CSF, Paris, France 22 Filed; 3, 197 3,226,662 12/1965 Walter et 3,311,839 3/1967 Rutulls ..330/4.9 [2]] Appl. No.: 8,315

Primary Examiner-Herman Karl Saalbach Assistant Examiner-C. Baraff [30] Foreign Application Priority Data Attorney-John W. Malley et al.

Feb. 17, 1969 France ..6903852 ACT In a filter comprising a plurality of cavities coupled to each 521 0.5. CI. ..333/73 w, 333/83 other by means of meta] probes, the cavities have respective [51] Int. Cl. .l'l03b 7/10 tuning rods which extend therein and whose crss secfina| Fi l 01' Se h 73 73 333/98 area is not constant throughout the whole of their length, so

that the filter bandwidth is made fairly constant within a large range of tuning frequencies.

3 Clains, 3 Drawing Figures PATENTED I972 3.680.012

' sum 2 or 2 ABA M /6 UVENT'OR JEN NOEL MOREA-U MICROWAVE BAND-PASS FILTER HAVING CONSTANT BANDWIDTH AS FILTER IS TUNED The present invention relates to microwave band-pass filters and in particular to band-pass filters which can be adjusted through a relatively wide frequency range.

It is well known to form band-pass filters for microwaves, with cascade-coupled resonant cavities, whose walls are of material of good electrical conductivity and which are the seat of oscillation at respective resonance frequencies, determined by the dimensions of the cavity.

It is also known to couple consecutive cavities by means of probes formed, for example, by a small metal rod passing, in insulating passage, through the wall common to the two cavities the resonance frequency of each cavity being adjusted by introducing into it a rod of dielectric material for example a quartz screw, the dielectric constant of which differs from that of the dielectric medium bounded by the cavity and which may be for example air or vacuum.

The resonance frequency of a filter of this type decreases with increasing penetration of this rod into the cavity, whilst theoretically the band-width maintains a constant relative value. However, in fact this law is affected by the parasitic capacitance between the coupling probe and the rod.

It is generally desirable that the filter band-width should maintain a constant absolute value throughout its range of adjustment.

This result has so far generally been obtained either by varying the intercavity coupling, or by laterally displacing the rod while its depth of penetration is varied. However, this results in a substantial complication of the control operations.

It is an object of this invention is to overcome these drawbacks.

According to the invention there is provided a band-pass filter for microwaves comprising: a plurality of cavities, metal probes extending through the walls common to two adjacent cavities and isolated therefrom, and tuning rods, respectively extending into said cavities, said rods having a dielectric constant which differs from that of the medium bounded by said walls and the cross-section of said tuning rods not being constant over the whole of their length.

For a better understanding of the invention and to show how it can be put into effect, reference will be made to the drawings accompanying the following description and in which:

F IG. 1 is an example of a band-pass filter, shown in section;

FIG. 2 is an explanatory diagram; and

FIG. 3 is an example of a control rod used in the band-pass filter of FIG. I in accordance with the present invention.

In FIG. 1 copper cavities I0, ll, 12 and 13, with their control rods 30, 31, 32 and 33 of quartz, are coupled with one another by metal probes l4, l5 and 16 located in respective insulating passages l7, l8 and 19. The terminal cavities l0 and 13 are coupled to the corresponding terminals and 2! by probes 22 and 23 passing through the insulating passages 24 and 25. The dielectric medium in the cavities is air.

Disregarding the shape of the multidiameter rods 30 to 33, the four cavity filter thus formed is known per se. It will be observed that for given metal probes, the width of the pass-band of the filter varies with the band-center frequency in accordance with a law which depends upon the diameter of the control rods.

In FIG. 2, the curves 40, 41 and 42 show, as a function of the resonance frequency expressed in Gc/s, and for conventional single diameter control rods whose diameter is of 10, 8 and 6 mm respectively, the variation in the corresponding band-width intervals, AB, expressed in Mc/s, in relation to the minimum band-width, which is used as a common reference for all the three curves.

The sections of the curves shown, which have a positive slope, correspond to an increase of the band-width as a function of frequency, a situation in which the decrease of the band-width due to the influence of the tuning rod on the variation of the intercavity coupling is weaker than the increase thereof resulting from the theoretical proportionalit between the band-width and the resonance frequency of t e cavity,

which increase is thus under-compensated by the action of the tuning rod; this is the case, for example, throughout the whole of curve 42.

In the contrary case i.e. in the case of the curve 40, there is excessive compensation, while there is optimal compensation in the central portion of the curve 41 and near one end of the ends of the other two curves.

This can be explained by the fact that the larger the rod diameter is, the more it increases the intercavity coupling by capacitive effect, with the result that optimum compensation is obtained at a frequency which rises with the diameter of the tuning rod.

The Applicant has found that a rod with a section which is not constant throughout the length thereof makes it possible to achieve the desired compensation within a wide frequency range and thus to obtain a constant band-width within this range.

An example of this kind of rod is shown in the sectional view of FIG. I where each of the tuning rods 30 to 33 comprises three dielectric sections 50, 51 and 52, the diameter of which increases from the free end of the rOd. The uppermost part of the other end of each tuning rod preferably includes a metal screw (not shown) running in a corresponding threaded cylindrical part in a conVentional manner, thus making it possible to vary the depth of penetration of the dielectric rod into the cavity, as indicated by the arrows 35. The free end of such a tuning rod is shown in perspective in FIG. 3, showing the three dielectric sections 50, SI and 52 with different diameters and different lengths DI and D2 in so far as sections 51 and 52 are concerned. The section 50 is fitted into the wall of the cavity in the manner indicated above or in any manner suitable for the vertical displacement thereof.

In one embodiment of the invention, which is mentioned, here by way of example, a rod of this kind was made of quartz and the diameters of the parts 50, 51 and 52 were respectively 10, 8 and 6 mm, D being equal to 9 mm and D to 6 mm.

A band variation of less than 10 percent was thus obtained in a 4-cavity tunable filter with a tuning range of 7.1 to 7.7 Gc/s.

Of course the invention is not limited to the embodiment described and shown which was given solely by way of example. In particular the section of the rod may vary progressively and not stepwise as in the case of FIG. I.

Filters of this kind find application in numerous fields and in particular in transmitter-receiver systems for telecommunication, e.g. in radio links.

What is claimed is:

l. A band-pass filter for microwaves comprising:

a plurality of cavities having walls,

metal coupling probes extending through the walls common to two adjacent cavities and isolated therefrom, and dielectric tuning rods respectively extending into said cavities to a variable depth and normally to said metal probes, said rods having a dielectric constant which differs from that of the medium bounded by said walls of said cavities and having a free end and means including the cross-section of said tuning rods increasing, at least stepwise, from the free end thereof for maintaining an approximately constant value of the bandwidth of said filter for different degrees of penetration of said rods into said cavities.

2. A filter as claimed in claim-l in which the cross-section of said tuning rods increases stepwise from the free end thereof with each step starting from said free end being longer than the preceding step.

3. A filter as claimed in claim I in which said rods are of quartz. 

1. A band-pass filter for microwaves comprising: a plurality of cavities having walls, metal coupling probes extending through the walls common to two adjacent cavities and isolated therefrom, and dielectric tuning rods respectively extending into said cavities to a variable depth anD normally to said metal probes, said rods having a dielectric constant which differs from that of the medium bounded by said walls of said cavities and having a free end and means including the cross-section of said tuning rods increasing, at least stepwise, from the free end thereof for maintaining an approximately constant value of the bandwidth of said filter for different degrees of penetration of said rods into said cavities.
 2. A filter as claimed in claim 1 in which the cross-section of said tuning rods increases stepwise from the free end thereof with each step starting from said free end being longer than the preceding step.
 3. A filter as claimed in claim 1 in which said rods are of quartz. 